Today sailed my I-S0nic 125 for the first time.
I used a RS5 9.0 and the stock 44 fin.
It felt a little small for the sail. Can you tell me what fin size is the best for this size of sail. I am about 172cm and 72kg.

For so far its a great board, have sailed before the Hypersonic 105,
and its a good new one for me.

How powered up was the sail? I found that to get the iSonic on a plane, I have to bear off the wind a bit and pump. By doing this, you don&#39;t put all the force on the fin (which would slip if I&#39;m interpretting your experience correctly). Once on a plane, you can head up on a beam or even upwind. I find I don&#39;t pump as much by pushing against the fin. Once on a plane, you will be able to push on the fin. I hope this makes sense. :-/ maybe someone can explain this better.

With my iS105, I use a 7.7 V8 with a 40cm fin or 34cm. If winds aren&#39;t strong enough to keep me powered up, the 40cm fin can even feel small. When this sail is powered up and things are really humming along, I pull my 34cm fin out. These are the fins that came with the iS105. But both fins work with this sail quite well if you learn this technique to pump onto a plane.

As O2bnme described, the faster you go, the more lift you develop for any fin size, and the iS tuning is more about top end speed, so typically the fin tuning remains a little bit smaller and the lift (fin performance) kicks in with extra board speed.

The stock sizes as provided (36 and 44cm for iS125) are selected for their maximum range, covering probably the middle "80%" of the iS125&#39;s range (typically..)

If you are underpowered on 9m (marginal conditions) then adding more fin ~48cm will help, (planing slow is always faster than not planing ) however as you come more into the power band the extra fin starts to detract from the top end potential of the iS, and lighter riders may even find the excessive lfin lift counterproductive to the overall trim of the iS (if the fin is too overpowering, it tends to disturb the rider&#39;s loading/drive down thru the mastfoot, which is a vital ingredient in making the iS really fly straight and true across chop)

Depending on your trim/tune, you may find the 44cm is better for powered/overpowered 9m conditions, esp with 72kg rider.

Of course, with 9m (c/w say 7m) the difference in comfortable to overpowered occurs across a smaller wind range, especially with lighter riders on smaller (c/w FW) boards like the iS125. In this case, the fin change down (to 44 if you are on 48) can help - and may avoid a sail change down, at least for another couple of kts of wind increase.

Hi Ricbra,
It should work very well, and since there a two distinct "types" of Hypersonic fins, you could use the different "tip outline" to advantage.
Need a tiny bit more power and early planing performance from the 44 cm Isonic fin..... use the older style (squared off tip) HS 44 cm.
Need a little less powerful fin and want to go faster, try the 44 cm HS fin with the SR6B tip profile. It&#39;s got a little less surface area, and a less "powerful" foil, so it&#39;s harder to get going, but faster and easier to control once you reach top speed.
There are lots of good fins out there, you just need to begin to understand how each different profile works for you (and against you as Ian suggests).
Hope this helps,

Definitely note Roger&#39;s comment about the SR6b version being faster.
On iS this is even more noticeable, as iS* is typically a low/er drag/higher efficency tuning combo than HS, which was tuned mostly with a high/er power with high/er drag combo.

It seems that iS hulls do not generate as much power as HSs so bigger fins go better (c/w HS).

The reason for my question was that I?ve just bougth an iS105 new but 2nd hand which came with the 34cm stock fin only.
I have a SR6b 38cm fin which I planned to use with 8.5 Severne OD sail on iS105. Probably not the best match from your answers but still ok I suppose.
Do you agree?
I also own a HS133 and untill I change it for iS133 by the end of this year, I could do with HSs fins.

Hi Ricbra,
It depends on how you rig the Overdrive, but I&#39;d suspect that an 8.5 m2 Overdrive would be a bit too much sail on the Isonic 105.
Should work well on the Isonic 115 and larger (although I never tried larger than 7.5 m2 on the &#39;06 Isonic 115 demo board I had) but I think it&#39;s going to be a little too big for the 105.
Give it a try though, and I think you are on the right track but a 38 cm SR6B is going to be quite small for a sail that large. I think your HS 133 44cm will be better, but if the wind comes up and you really get the 8.5 OD downhauled enough to look and sail like a full on race sail, then maybe it will work.
Hope this helps,

First of all thanks for the clues they were helpfull indeed.
I had the opportunity to sail my iS105 for the first time this past week.
Conditions called for 6.5 and 8.5 for my size ( 66Kg, 1.7m ).
However I sailed with the wrong sail each time. I mean when it was 8.5 I was on 6.5 and vice versa.
However I have to say that this is another excelent product from *B. Congratulations to all!

Sailing iS105 with 8.5 Severne OD rigged for high winds is definitely OK. I used HS 42cm stock fin on both ocasions. II found that it sailed fairly well upwind and had no spinouts though I never pushed to check the limit.
I was overpowered and get tired relatively soon. But other than that tacking was the only issue with such sail size. I was surprized to notice that it planes early for its size and IMHO is less demanding to sail and jybe c/w HS133. It was easily under control and its real speed is not easily noticed because of that.
It is a very different animal to HS and although I love my HS133 I definitely will change it for an iS133

I&#39;m pretty happy and can&#39;t wait to sail it with the right sail next time.